Compression and magnetics

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Saltspringer
Posts: 12
Joined: 27 Jun 2016 17:33
Location: BC, Canada
Location: Saltspring Island, BC, Canada

Compression and magnetics

Post by Saltspringer »

In order to make sure I have the best chance of starting my newly purchased 1981 500 (with 35,000 km on the clock) I have begun to check the obvious.

I did a compression check last night and found that I had 180psi on both cylinders, with the throttle shut. I am glad to see that they were both the same but a bit surprised that the numbers were quite that high. Good news I guess, but has any one else done a compression check and found similar results?

I suspect that the stator and rotor are getting tired and might have the stator rewound. Does anyone have a resistance recommendation, below which a rewind is advisable?

Finally, how to you determine if the rotor magnetism has declined to a point where remagnetising is a good idea?

Thanks in advance for any comments and suggestions.
Andy Higgs

Saltspring Island, BC, Canada

1981 Moto Morini 500
1945 WL 45 Harley Flathead
1960 BMW R60/2
1972 BMW R60/5

Owned in the past various bikes including a Ducati Pantah and a Laverda Jota
3potjohn
Posts: 1243
Joined: 02 Jun 2007 13:58
Location: Devon

Re: Compression and magnetics

Post by 3potjohn »

Resistance across the green wire from the generator to fuse board ,in my case on a 350 was 186 ohms. The bike ran but was not an easy start. After a rewind it went towards 300 ohms which fixed that issue but I understand that around 250 is sufficient.What is your reading?
Saltspringer
Posts: 12
Joined: 27 Jun 2016 17:33
Location: BC, Canada
Location: Saltspring Island, BC, Canada

Re: Compression and magnetics

Post by Saltspringer »

Hi John

Looking through the previous owners scribbled notes a few years ago there is mention of a reading of 220 ohms which sounds like it was moving to less than optimal. I am going to check mine today but suspect that it is going to be lower again...

Thanks for the benchmark with new windings that is really helpful.

I will let you know what the stator reads.

Cheers

Andy
Andy Higgs

Saltspring Island, BC, Canada

1981 Moto Morini 500
1945 WL 45 Harley Flathead
1960 BMW R60/2
1972 BMW R60/5

Owned in the past various bikes including a Ducati Pantah and a Laverda Jota
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72degrees
Posts: 1549
Joined: 31 Aug 2007 21:24
Location: West Midlands

Re: Compression and magnetics

Post by 72degrees »

300 ohms is the 'gold standard' but Evguru will be along in a minute to point out that resistance isn't the whole story. I like to test them on the bike by voltage output (AC) at kickstart speed.

When I took a dubious rotor to NLM a few years ago they just 'tested' it by seeing how strongly the magnets attracted a spanner. I couldn't suggest a calibrated attraction value though ;)
EVguru
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Location: Luton
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Re: Compression and magnetics

Post by EVguru »

When doing a compression test, you want the cylinders to be full of air, so a shut throttle doesn't make sense, the result will be quite affected by how fast you turn the engine over.

The only thing that matters with the ignition winding is the voltage produced.

The voltage is dependent on the strength of the magnetic field, the number of turns on the winding and the speed at which the magnetic field passes through the winding (how fast the engine is turning over).

I have dismantled and rewound enough coils to know that an OEM 300 ohm coil is a rare beast indeed and they are the most likely to suffer insulation failure resulting in shorted out turns. The shorted turns will result in less voltage and a lower than original resistance is a diagnostic symptom of this.

The majority of coils use a thicker wire gauge that means there simply isn't enough room on the bobbin to wind a coil that would result in a resistance of more than about 250 ohm. I have seem more 'good' coils with a resistance of close to 220 ohm than anything else.

If the bike starts easily, then there's simply no reasons to concern yourself with the resistance.

A good rotor should take a fair tug to get it off the stator. If it's generally weak, or the magnets vary in strength, then it would probably benefit from re-magnetising. NLM can offer the service and so can I. I usually have the magnetiser on the club stand at the Stafford shows.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
Saltspringer
Posts: 12
Joined: 27 Jun 2016 17:33
Location: BC, Canada
Location: Saltspring Island, BC, Canada

Re: Compression and magnetics

Post by Saltspringer »

Hi Paul

Thanks for the detailed reply, much appreciated.

With regard to the compression test; for both cylinders I kicked the bike until the reading appeared to stabilize. The bike will start but it is a real struggle to get it going using the kick starter. Once it runs the bike sounds good, no knocks or rattles and no smoke. It will idle but the carbs obviously need a bit of setting up.

I had a unplanned look inside the front cylinder today after all three of the bolts holding the rocker cover on suddenly went soft and pulled their threads... I had a Helicoil kit from my Laverda days so while the head was off I checked the valves and bores. All looked good.

I also checked the stator and found I had a reading of 165 ohms. When I pulled the rotor is is still quite magnetic but the coils seem to have had a hard life, and one looks to have been replaced. I think that I will try and get them refurbished in Vancouver unless the collective wisdom here on the list has an alternative suggestion.

Many thanks for the advice, all very helpful.

Andy
Andy Higgs

Saltspring Island, BC, Canada

1981 Moto Morini 500
1945 WL 45 Harley Flathead
1960 BMW R60/2
1972 BMW R60/5

Owned in the past various bikes including a Ducati Pantah and a Laverda Jota
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Ming
Posts: 798
Joined: 01 Aug 2014 16:32
Location: France
Location: Central France

Re: Compression and magnetics

Post by Ming »

The ignition coil is relatively simple to rewind, if a bit fiddly. Have a look on the Dutch Moto Morini site, there is an article on there about it. (motomoriniclub.nl)
If you want to remagnetise your rotor, try a scooter mechanic, they have a similar system.
Saltspringer
Posts: 12
Joined: 27 Jun 2016 17:33
Location: BC, Canada
Location: Saltspring Island, BC, Canada

Re: Compression and magnetics

Post by Saltspringer »

That is a great link for the Stator rebuild Ming, much appreciated. I think that I could do this if I had to!

Cheers

Andy
Andy Higgs

Saltspring Island, BC, Canada

1981 Moto Morini 500
1945 WL 45 Harley Flathead
1960 BMW R60/2
1972 BMW R60/5

Owned in the past various bikes including a Ducati Pantah and a Laverda Jota
rossguzzi
Posts: 433
Joined: 23 Jan 2019 14:21
Location: Perth W/Australia

Re: Compression and magnetics

Post by rossguzzi »

So what should the compression test show in PSI ?
180 sounds very high ??
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